Many processes exist wherein a hot gaseous medium is produced which contains particulate material that must be separated from the gaseous medium, either to prevent pollution, to remove hazardous material, or to reclaim the particulates for further use. Numerous such processes use fluidized bed reactors which produce a hot, corrosive gaseous mixture from which particulate material must be removed.
In the formation of zirconium tetrachloride from zirconia sand, for example, a fluidized bed can be used to chlorinate zirconia and extract zirconium from zirconia sand. Zirconia sand and coke are fluidized by nitrogen and chlorine gases at low pressure and high temperature producing zirconium tetrachloride as the useful product gas. As with all fluidized bed systems, some reactant is lost as small unreacted solid particulates of sand and coke are carried over by the process gas into downstream equipment. To minimize this reactant loss, the upper section of the reactor is usually enlarged in size to reduce process gas velocity which minimizes the particle size of particulates carried out of the reaction chamber. This enlarged section is called the disengagement section. To further improve operating efficiency, cyclones can be installed in or above the disengagement section. In the last ten years, a considerable amount of development effort has been expended developing an alternative to cyclones for control of particulates in high temperature fluid bed reactor processes.
One type of system for the removal of particulates from a hot gas stream is described in U.S. Pat. No. 4,343,631 to David F. Ciliberti, and is assigned to the assignee of the present invention. In that system, a compact, cross flow filter apparatus is described containing a plurality of relatively small cross flow ceramic filter elements or blocks, and ducts or tubes that are nested in an arrangement to form a high density filter system.
In place of block-like cross flow filter elements, it has been proposed in the prior art to use ceramic candle filter elements in high temperature gas filtration techniques. Such ceramic candles can be formed from silicon carbide or other ceramics which have mineral fiber flakes therein, and are hollow tubuar elements, with the gas passed through the walls of the tubular element, such that particulates collect on the outer walls, and the cleaned gas discharged from the hollow of the tubular member. Pulse, or blowback gas can be supplied from a plenum in a head of a vessel containing the candles to permit cleaning of portions of the candles in the vessel in sequence.
U.S. Pat. No. Re. 24,954 issued Mar. 28, 1961 to Church shows apparatus for filtering solids from gas-solid suspensions and shows a chamber inside top cover 28 which chamber apparently provides some preheating of his blowback pulse. U.S. Pat. No. 3,325,979 issued June 20, 1967 to Smith shows a filter bag system in which, at column 3, lines 37- 40, it is noted that "if desired, the source of reverse air may be the discharge of the fan, rather than the atmosphere, to provide warmer gases and prevent sudden chilling of the bags." Russian Patent 507,337 published Aug. 25, 1976, to Kabanov shows a filter system with a pulsed blowback and also with a periodic introduction of warm air to oxidize soot and to carbon monoxide and carbon dioxide to free the deep pores of the filter elements of soot.
It is an object of the present invention to provide a blowback type filter cleaning process for hollow, tubular filter elements for the continuous filtering of solid particulate material from hot gas streams, particularly corrosive gas streams.
It is another object of the present invention to provide a filter cleaning process for use in continuous filtering particulate material from hot gas streams which contain gaseous constituents that are subject to condensation or solids formation during the separation of the particulate material through cooling of the gas stream below a particular temperature value (i.e., at temperatures between 100.degree. C. and the filter operating temperature).